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1 Band Edge Energies and Excitonic Transition Probabilities of Colloidal CsPbX 3 (X= Cl, Br, I) Perovskite Nanocrystals Vikash Kumar Ravi, Ganesh B. Markad, *, Angshuman Nag,, * Department of Chemistry and Centre for Energy Science, Indian Institute of Science Education and Research (IISER), Pune, , India. *Corresponding authors s: AN: angshuman@iiserpune.ac.in GBM: ganesh.markad@iiserpune.ac.in. Supporting Information Experimental Section: Chemicals: Cesium carbonate (Cs 2 CO 3, 99.9%, Aldrich), lead (II) bromide (PbBr 2, %, Aldrich), lead (II) chloride ( PbCl 2, 99.99%, Aldrich), lead (II) iodide (PbI 2, 99.99%, Aldrich), oleic acid (OA, 90%, Aldrich), oleylamine (OAm, technical grade 70%, Aldrich), 1-octadecene (ODE, technical grade 90%, Aldrich), toluene (anhydrous 99.8%, Aldrich), trioctylphosphine (TOP,98%, Aldrich), ethyl acetate(99.5%, Rankem), tetrabutylammonium perchlorate (TBAP, 99.0%, Aldrich), tetrabutylammonium hexafluorophosphate (TBAPF 6, 98% Aldrich), S 1

2 acetonitrile (anhydrous, 99.8%, Aldrich), dimethyl sulfoxide (anhydrous, 99.9%, Aldrich), ferrocene (98%, Aldrich). Preparation of Cs-oleate: g (1.25 mmol) Cs 2 CO 3, 1.25 ml OA and 15 ml ODE were mixed in a 50 ml three-necked round bottom flask and the reaction mixture was kept under vacuum for 15 min at 110 C, followed by purging with N 2 for 10 min along with magnetic stirring. This method of alternate application of vacuum and N 2 was repeated 3 times to achieve the removal of moisture and O 2 from the reaction mixture. The temperature of the reaction mixture was then increased to 130 C, and was continued heating until the Cs 2 CO 3 dissolved completely giving a clear solution. This Cs-oleate solution in ODE was used as cesium precursor for the synthesis of CsPbBr 3 nanocrystals (NCs). Synthesis of CsPbBr 3 nanocrystals: Colloidal CsPbBr 3 nanocrystals were synthesized by following a method similar to ref ml dried ODE and g (0.752 mmol) PbBr 2 were taken in a 50 ml three-necked round bottom flask. The mixture was degassed (under alternate vacuum and nitrogen) at 120 ⁰C for 60 minutes along with magnetic stirring. Dried OA and OAm, each 2.0 ml, was added to the mixture at 120 C having N 2 flow. After complete dissolution of PbBr 2 in ODE, the reaction temperature was increased to 190 C. Separately prepared Cs-oleate (0.1 M, 1.6 ml) solution in ODE, pre-heated to 100 C, was swiftly injected to the reaction mixture. The reaction mixture became greenish and the reaction was stopped by dipping the reaction flask into an ice bath. The synthesized CsPbBr 3 NCs were precipitated by adding ethyl acetate at room temperature and then centrifuged at 7000 rpm. Finally, the precipitate of the NCs was redispersed in 20 ml toluene and was used as stock solution for anion exchange reaction. S 2

3 Anion Exchange of CsPbBr 3 NCs: Anion exchange reactions were carried out following ref 3 20 ml colloidal CsPbBr 3 NCs in toluene (as discussed above) was divided into 5 parts having 4 ml each. For preparation of CsPbCl 3 and CsPb(Cl/Br) 3, PbCl 2 in different concentration (0.220 mmol for CsPbCl 3, and mmol for CsPb(Cl/Br) 3 ) were mixed with ODE (4 ml) in 25 ml three- necked round bottom flask and was degassed and dried by applying alternate vacuum and N 2 flow for 1hr.at room temperature. Dried OA (0.5 ml) and OAm (0.5 ml) along with TOP (1 ml) were injected at 200 C under N 2 flow to make PbCl 2 completely soluble. The reaction mixture was then allowed to cool down to room temperature and then CsPbBr 3 NCs stock solution (4 ml) was injected to give anion exchanged CsPbCl 3 and CsPb(Cl/Br) 3 NCs. Similarly, for preparation of CsPbI 3 and CsPb(Br/I) 3, and mmol PbI 2 were used in ODE (4 ml), and dried OA (0.2 ml) and OAm (0.2 ml) were added to reaction mixture at 120 C under N 2 environ to make PbI 2 soluble. The reaction mixture was then allowed to cool down to room temperature and 4 ml CsPbBr 3 NCs stock solution was injected to give anion exchanged CsPbI 3 and CsPb(Br/I) 3 NCs. NCs were precipitated by adding ethyl acetate (1:1 v/v) and then centrifuged at 7000 rpm for 10 min. The supernatant was discarded and the wet precipitate was re-dispersed in toluene and used for further characterization. Characterization of CsPbX 3 Nanocrystals: Perkin Elmer, Lambda-45 UV/Vis spectrometer was used for recording UV-visible absorption spectra. Steady state photoluminescence (PL) and PL decay measurements were carried out using FLS 980 (Edinburgh Instruments). Powder x-ray diffraction (XRD) data were recorded using a Bruker D8 Advance x-ray diffractometer using Cu Kα radiation (1.54 Å). Transmission electron microscopy (TEM) studies were carried out using a JEOL JEM 2100F field emission transmission electron microscope at 200 kv. Thermogravimetric analysis (TGA) measurements were carried out using Perkin Elmer STA S 3

4 6000. The samples were heated from 30 C to 800 C at the heating rate of 10 C/min. Scanning electron microscopy (SEM) and energy dispersive x-ray spectroscopy (EDAX) measurements were performed on Zeiss Ultra Plus SEM instrument. Dynamic light scattering (DLS) data were obtained using a Nano-ZS90 from Malvern Instruments, U.K. Calculation of Molar Extinction coefficient (ε): Each solution of CsPbX 3 NCs were divided into two equal parts and the NCs were then precipitated by adding ethyl acetate and centrifuging at 7000 rpm at room temperature. First part of nanocrystals was again dispersed in a known volume of toluene for UV-visible absorption measurements, and the second part was vacuum dried in order to obtain the weight of NCs. In order to subtract the weight of organic capping ligands from the total weight of organic capped NCs, TGA data were recorded showing ~10% weight from organic capping ligand. Therefore, 90% of the total weight of organic capped NCs was used to calculate the molar concentration of inorganic core NCs absorbing the visible light. Considering cubic shape of NCs and using size obtained from TEM, weight of a single NC was calculated using the formula, m = d/v, where m is the mass of each nanocrystal, d is density of bulk CsPbX 3 (CsPbCl 3 = 4.24 g/cm 3, CsPbBr 3 = 4.86 g/cm 3, CsPbI 3 = 5.07 g/cm 3 ), 4 and v is the volume of a NC (v = a 3 where a = average edge-length of a nanocube obtained from TEM).Densities of alloyed CsPb(Cl/Br) 3 was extracted from that of their end members CsPbCl 3 and CsPbBr 3 after weighing with the composition of the alloy. Likewise, density of CsPb(Br/I) 3 were extracted from their end-members CsPbBr 3 and CsPbI 3. Now, the number of NCs in the sample was obtained by dividing the total weight of inorganic part of the NC sample with weight of a single NC. These numbers of NCs were then converted to mole of NCs by dividing with Avogadro s number (N A ). Molar concentrations (C) of the NCs were then calculated from knowledge of volume of toluene used in the dispersion. S 4

5 For determination of ε, the absorption spectrum of dilute solution of NCs was obtained having different concentrations. ε was then obtained using Beer-Lambert Law : A = ε C L, where, A is the absorbance, and L is the optical path length (1 cm) taken from cuvette dimension. In order to account for variations in size distribution causing broadening of absorption peak, the obtained molar extinction coefficient value was multiplied by a normalizing factor E HWHM / E x following ref. 5, where E HWHM (ev) was the fitted half-width-half-maximum of first absorption peak and E x was selected as 0.06 ev as it was average half-width-half-maximum of all the samples used here for the measurement. Electrochemical Measurements: Cyclic Voltammetry (CV) measurements were performed with the help of PAR Potentiostat/Galvanostat (model PARSTAT 2273). A commercial Pt disk electrode (CHI Instruments, USA, 2-mm diameter), Ag wire, and Pt-wire loop were used as working, quasi reference, and counter electrodes, respectively. Prior to use, the working electrode was polished over 0.5 μm alumina powder and rinsed with Milli-Q water. Further it was pretreated electrochemically with 0.5 M H 2 SO 4 by cycling the potential for several times between 1.2 V and V (scan rate of 1 V s -1 ). Electrochemical measurements were done in a 5-neck glass electrochemical cell. After fixing all the electrodes, g of TBAP was transferred into the cell and it was then air tight inside the glove-box. Cell was then taken out of the glove-box and TBAP was dried under vacuum at 80 C in the oil bath for a 1 hr, cell was then allowed to cool down to room temperature naturally and vacuum was released by high purity nitrogen gas. 5 ml acetonitrile and toluene mixture (1:4 v/v) was injected in the cell through the septum and electrochemical measurements were carried out with slight positive pressure of N 2 gas in air tight cell. At the end of each set of experiments, the potentials were calibrated with respect to the normal hydrogen electrode (NHE) using ferrocene S 5

6 as an internal standard. 6 Formal potential of ferrocene/ferrocenium redox couple in 1:4 mixture of acetonitrile/toluene is estimated experimentally using Ag/AgNO 3 (10 mm in acetonitrile) reference electrode. Formal redox potential of ferrocene/ferrocenium redox couple in 1:4 mixture of acetonitrile/toluene is 0.78 ev (See Figure S9 in SI). Potentials were reported with respect to NHE throughout the manuscript. Figure S1: Digital photographs of colloidal dispersion of CsPbX 3 NCs captured under ambient light with various halide compositions obtained after the anion exchange reaction. S 6

7 Figure S2: XRD patterns of CsPbX 3 NCs with overlay of reference patterns of respective bulk materials. S 7

8 (a) (b) Figure S3: Representative TEM image recorded on (a) CsPbCl3 NCs and (b) CsPbBr3 NCs. S 8

9 Figure S4: SEM image of CsPbCl 3 NCs. S 9

10 Figure S5: SEM image of CsPb(Cl/Br) 3 NCs. S 10

11 Figure S6: SEM image of CsPbBr 3 NCs. S 11

12 Figure S7: SEM image of CsPb(Br/I) 3 NCs. S 12

13 Figure S8: SEM image recorded on CsPbI 3 NCs. S 13

14 Figure S9: Cyclic voltammograms recorded for ferrocene/ferrocenium redox couple in 50 mm TBAP solution in 1:4 v/v mixture of acetonitrile and toluene. Scan rate used is 50 mv/s. S 14

15 Figure S10: (a) UV-visible absorption, (b) PL spectra, and (c) XRD patterns of CsPbBr 3 NCs before and after the 10 cycles of CV measurements. Inset of (c) shows the digital photograph of CsPbBr 3 NCs solution in toluene captured before and after CV measurements (after CV measurement, the nanocrystal solution was precipitated and redispersed in toluene). There is no noticeable change in peak position in UV-Vis and PL spectra of the nanocrystal solution obtained before and after the CV measurements however, PL peak intensity decreased somewhat after the CV measurements. Decrease in PL intensity is probably because of minor agglomeration and/or presence of different electrolyte systems. Similar XRD patterns are observed for both samples before and after CV measurements, confirming the crystal structure remained intact during the CV experiment. Also widths of XRD peaks remained unaltered before and after the CV measurements, again suggesting that the crystal size does not change during the CV measurement. All these results confirm that the NCs are stable during the CV measurements. S 15

16 200 nm Figure S11: Representative TEM image recorded after casting CsPbBr3 NCs on TEM grid from their dispersion in 1:4 v/v mixture of acetonitrile and toluene. S 16

17 Figure S12: (a) DLS data of CsPbBr 3 NCs dispersion in toluene and acetonitrile: toluene 1:4 v/v mixture at different times after making the dispersion. (b) Cyclic voltammograms recorded on CsPbBr 3 NCs in acetonitrile and toluene 1:4 v/v mixture at different times after addition of sample in electrochemical cell. For clarity, magnified regions of anodic and cathodic peaks are shown in (c) and (d) respectively. To compare voltammograms at different time, after each measurement working electrode was cleaned by polishing it over 0.5 µm alumina powder. Scan rate used was 50 mv/s. S 17

18 Figure S13: Cyclic voltammograms recorded on CsPbBr 3 NCs, oleic acid, oleylamine and Pboleate in 50 mm TBAP solution in 1:4 v/v mixture of acetonitrile and toluene. Scan rate used is 50 mv/s. S 18

19 Figure S14: Cyclic voltammograms recorded on CsPbBr 3 NCs with different scan directions in 50 mm TBAP solution in 1:4 v/v mixture of acetonitrile and toluene. Scan rate used is 50 mv/s. S 19

20 Figure S15: First cycle of cyclic voltammograms recorded on CsPbBr 3 NCs with different scan rates in 50 mm TBAP solution in 1:4 v/v mixture of acetonitrile and toluene. Inset shows plot of peak current of A 1 and C 2 versus square root of scan rate where solid line indicate linear fit. (To compare voltammograms at different scan rates, after measurement at each scan rate working electrode was cleaned by polishing it over 0.5 µm alumina powder and then measurements at another scan rate were done same NCs dispersion. Also for all the scan rates first cycle of measurement is presented). S 20

21 Figure S16: Cyclic voltammograms recorded on CsPbBr 3 NCs in different solvent/electrolyte media. Scan rate used was 50 mv/s. TBAP: tetrabutylammonium perchlorate; TBAPF 6 : tetrabutylammonium hexafluorophosphate; NaClO 4 : sodium perchlorate; ACN: acetonitrile; and DMSO: dimethyl sulfoxide. S 21

22 Figure S17: Comparison of band edge positions of CsPbX 3 NCs estimated from our CV results with selected III-V and II-VI semiconductors in bulk state. Results for III-V and II-VI semiconductors are taken from ref 7,8. S 22

23 Figure S18: Band edge positions of our CsPbX 3 NCs with respect to different hole and electron transport materials used for light emitting diode device preparation, values adopted from ref 9. S 23

24 Figure S19: PL decay of CsPbX 3 NCs at PL peak wavelength. Fitting parameters are given in Table S2. S 24

25 Table S1: Optical and electrochemical band gap, VBM and CBM energy levels versus NHE and vacuum of CsPbX 3 samples NCs Optical gap (ev) Electrochemical gap (ev) VBM (V) vs NHE CBM (V) vs NHE VBM (V) vs vacuum CBM (V)vs vacuum CsPbCl CsPb(Cl/Br) CsPbBr CsPb(Br/I) CsPbI Table S2: PL Decay lifetime of CsPbX 3 NCs obtained from Figure S19 at their emission peak wavelength. Sample a 1 τ 1 (ns) a 2 τ 2 (ns) τ av (ns) CsPbCl CsPb(Cl/Br) CsPbBr CsPb(Br/I) CsPbI S 25

26 References: 1. Protesescu, L.; Yakunin, S.; Bodnarchuk, M. I.; Krieg, F.; Caputo, R.; Hendon, C. H.; Yang, R. X.; Walsh, A.; Kovalenko, M. V. Nanocrystals of Cesium Lead Halide Perovskites (CsPbX 3, X = Cl, Br, and I): Novel Optoelectronic Materials Showing Bright Emission with Wide Color Gamut. Nano Lett. 2015, 15, Swarnkar, A.; Chulliyil, R.; Ravi, V. K.; Irfanullah, M.; Chowdhury, A.; Nag, A. Colloidal CsPbBr 3 Perovskite Nanocrystals: Luminescence beyond Traditional Quantum Dots. Angew. Chem., Int. Ed. 2015, 54, Nedelcu, G.; Protesescu, L.; Yakunin, S.; Bodnarchuk, M. I.; Grotevent, M. J.; Kovalenko, M. V. Fast Anion-Exchange in Highly Luminescent Nanocrystals of Cesium Lead Halide Perovskites (CsPbX 3, X = Cl, Br, I). Nano Lett. 2015, 15, Liu, Z.; Peters, J. A.; Stoumpos, C. C.; Sebastian, M.; Wessels, B. W.; Im, J.; Freeman, A. J.; Kanatzidis, M. G. Proc. SPIE 2013, 8852, 88520A. 5. Jasieniak, J.; Smith, L.; van Embden, J.; Mulvaney, P.; Califano, M. Re-examination of the Size-Dependent Absorption Properties of CdSe Quantum Dots. J. Phys. Chem. C 2009, 113, Bard, A. J.; Faulkner, L. R. Electrochemical Methods: Fundamentals and Applications, John Wiley and Sons, NewYork Wei, S. H.; Zunger, A. Calculated Natural Band Offsets of All II-VI and Ill-V Semiconductors: Chemical Trends and the Role of Cation d Orbitals. Appl. Phys. Lett. 1998, 72, Reiss, P.; Protiere, M.; Li, L. Core/Shell Semiconductor Nanocrystals. Small 2009, 5, Zhang, X. Y.; Lin, H.; Huang, H.; Reckmeier, C.; Zhang, Y.; Choy, W. C. H.; Rogach, A. L. Enhancing the Brightness of Cesium Lead Halide Perovskite Nanocrystal Based Green Light-Emitting Devices through the Interface Engineering with Perfluorinated Monomer. Nano Lett. 2016, 16, S 26